Ringing signal discriminators (referred to below as ring discriminators) detect the presence of ringing signals from other signals that may be present on a telephone line, such as noise, dual tone multifrequency (DTMF) signals, crosstalk from other telephone lines, control signals of various types, harmonics generated from various signals and noise such as control signals, etc.
In certain jurisdictions a ringing detector is required to react to any signal of at least a certain amplitude within the frequency band of 23 and 54 Hz as a true ringing signal, and outside of the band of 18 and 64 Hz as a false signal. 18-24 and 45-64 Hz zones are insensitive zones (see FIG. 1). Frequencies 20 and 60 Hz are chosen roughly in the middle of the insensitive zones as two digital thresholds between which the existence of a ringing signal is considered to be true. The reason for that is that the factor 3 (60/20) is easy to detect digitally.
A ringing signal is thus considered to have a frequency of between 20 Hz and 60 Hz. Only +/- 8.3% separates 64 Hz, a false ringing frequency threshold, and 54 Hz, which is within the band and which is a true ringing frequency threshold. It has been found that circuits that use analog signals are slow in detecting the ringing signal, requiring several ringing cycles to be able to detect the ringing signal, and are sensitive to tolerance variations in the components which make up the ringing signal.
Slow ringing signal detection can result in lost calls due to a receiving party taking a handset off hook prior to completion of ringing signal detection. Ringing signals cannot be recognized in this case though they exist, and connection of the call cannot be made. Sensitivity to tolerance variations can result in in-band ringing signals not being detected or out-of-band signals being mistaken for in-band ringing signals.